In the field of radiotherapy, an ionization chamber is generally used for measuring ionizing radiation. An ionization chamber is a measuring device which collects charges, generated at the time of ionization of air contained in a minute volume by radiation, by means of a high voltage of several hundreds of volts, and assesses a dosage from an amount of charges which are collected.
On the other hand, solid-state detectors are also used as a means for measuring the dosage. JP 2004-526155 A, for example, discloses a detector formed by a combination of a scintillator and an optical fiber, in which the scintillator emits light by an ionization action of radiation. The detector then measures the amount of light emission to thereby assess the dosage, based on a proportional relationship between the amount of light emission and the ionization. Here, a photoelectron multiplier tube is used for measurement of the amount of light emission, which is then converted to electric current.
Further, WO 2005/008287 A discloses technology related to counting of thermal neutron flux using a scintillator.
In JP 2004-526155 A described above, when the amount of electric current which is converted from the amount of light emission is extremely small, it is necessary to take measures such as (i) increasing the amount of light emission; (ii) increasing the amplification factor after conversion of electric current; (iii) employing a high-precision ammeter; and so on.
While the measures for increasing the amount of light emission (above (i)) can be achieved by increasing the size of the scintillator, a large size scintillator is disadvantageous in measurement in a very small region. Further, the above measures (ii) and (iii) would require expensive electrical equipment.